1. Field of the Invention
This invention relates to modified tissue laminates, and more specifically to a laminate reinforced with a center ply web to improve its tear strength.
2. Description of the Prior Art
Laminates have been prepared by impregnating a central reinforcing web having an open mesh with an adhesive and then placing tissue layers on the upper and lower surfaces of the web. The tissue layers become bonded to the reinforcing web. These tissue laminates are intended to be used as inexpensive substitutes for conventionally woven cloth fabrics. It is a goal of those skilled in the art to achieve clothlike properties which are characteristic of woven fabrics such as drape, bend, strength and absorbency, while at the same time supplying a cost saving substitute.
Laminate tear strength can be attributed to the central reinforcing web. This web can be knitted, woven or nonwoven. If a nonwoven web is used it may be formed by spunbonding, fiber entanglement or needle punching. The fibers used in reinforcing webs may be composed of a natural or synthetic substance such as cotton, rayon, polyamide, polyester, polyolefins, or blends of such substances. The particular fiber selected for reinforcement and the construction of the web are significant factors in the overall strength properties of the laminate. Another goal of those skilled in the art is to produce laminates which have good delamination resistance while maintaining the excellent tear strength of the central reinforcing web.
In addition to tear strength a laminate can also be characterized by flexibility and absorbency. These properties have been generally attributed to the characteristics of the tissue layers, center ply and/or the type and quantity of adhesive used for bonding the layers in the laminate.
Tissue laminates having good tear strength and offering clothlike properties have been produced with varying degrees of success. Some laminates have been successfully substituted for conventional woven fabrics. Prior workers have modified laminate properties by varying the type and quantity of adhesive, the construction of the reinforcing web and/or the type of tissue layer. U.S. Pat. No. 4,142,017, to William A. Blackburn and Roy D. Goodwin, issued Feb. 27, 1979, reported that "a soft and drapable product can be attained with lesser amounts of adhesive but delamination of the individual layers will generally occur. On the other hand, a higher concentration of adhesive will prevent delamination but there will be a sacrifice in laminate aesthetics."
In general, workers in the field have recognized that laminates, especially tissue laminates, having adequate strength and capable of resisting tear would be commercially attractive as alternative materials for conventionally woven and/or knitted fabrics. It has been reported that laminates having the property of resistance to tear can be obtained by using a reinforcing scrim having strong individual fibers capable of substantially unrestricted movement when the laminate is stressed. U.S. Pat. No. 3,629,047, to Robert W. Davison, issued Dec. 21, 1971, recognized that the strength of laminates and particularly tear strength can be enhanced by preventing or minimizing the adherence of the central reinforcing scrim to the outer layers.
Although the prior art reports that the fibers constituting the reinforcing web must have substantial freedom of movement; the accomplishment of such a result has proven to be elusive in the actual fabrication of flexible laminates and particularly tissue laminates. Blackburn et al. '017 taught that many well-known adhesives such as water-insoluble polymer dispersions (latexes) have caused adhesion to take place between the fibers of reinforcing webs and the outer tissue layers. This, according to Blackburn et al., has resulted in restricting fiber movement within the laminate with a resultant decrease in laminate strength properties, particularly tear strength. While Blackburn et al. obtained high tear strengths with their development, by so reducing the adherence of the inner fibers to the outer tissues and adhesive, the amount of latex used to prevent delamination of the outer tissues must be increased so much that the absorbent properties and flexibility of the laminate are substantially reduced.
Fabrics, bonded with thermoplastic adhesives, have been disclosed. U.S. Pat. No. 3,679,347, to Leonard T. Lehmann, issued Oct. 10, 1972, discloses a process of forming air-pervious reinforced nonwoven fabrics which comprises heating a nonwoven web in contact with a thermoplastic net material which has uniformly spaced thick portions connected by thinner strands. The temperature is sufficiently high to melt the net material, causing the strands to draw back into the thick portions making uniformly spaced spots of melted thermoplastic material. Lehmann teaches that the best results are obtained when the thermoplastic encircles and entraps the fibers--Column 2, lines 14-19. This patent is incorporated herein by reference.
Similarly, U.S. Pat. No. 3,900,632, to James E. Robinson, issued Aug. 17, 1975, discloses laminates of tissue and a random laid continuous filament web bonded with a discontinuous adhesive pattern which apparently completely penetrates the center ply and bonds the outer plies to said center ply (See FIGS. 1 and 2 of that patent). While plastisol adhesives are preferred, other adhesives such as hot melts, latexes, or thermoplastic fibers are disclosed. When using this method to produce a laminate, it has now been discovered that the penetration of the adhesive through the central reinforcing ply causes excess interfiber bonding which decreases tear strength.